s and the Permian, there is little trace of it before the
Coal-forests (after the Cambrian). However that may be, there was a
considerable lessening of the carbon-dioxide of the atmosphere, and
this in turn had most important effects. First, the removal of so much
carbon-dioxide and vapour would be a very effective reason for a general
fall in the temperature of the earth. The heat received from the sun
could now radiate more freely into space. Secondly, it has been shown by
experiment that a richness in carbon-dioxide favours Cryptogamous plants
(though it is injurious to higher plants), and a reduction of it would
therefore be hurtful to the Cryptogams of the Coal-forest. One may
almost put it that, in their greed, they exhausted their store. Thirdly,
it meant a great purification of the atmosphere, and thus a most
important preparation of the earth for higher land animals and plants.

The reader will begin to think that we have sufficiently "explained"
the Permian revolution. Far from it. Some of its problems are as yet
insoluble. We have given no explanation at all why the ice-sheets, which
we would in a general way be prepared to expect, appear in India and
Australia, instead of farther north and south. Professor Chamberlin,
in a profound study of the period (appendix to vol. ii, "Geology"),
suggests that the new land from New Zealand to Antarctica may have
diverted the currents (sea and air) up the Indian Ocean, and caused a
low atmospheric pressure, much precipitation of moisture, and perpetual
canopies of clouds to shield the ice from the sun. Since the outer polar
regions themselves had been semi-tropical up to that time, it is very
difficult to see how this will account for a freezing temperature in
such latitudes as Australia and India. There does not seem to have been
any ice at the Poles up to that time, or for ages afterwards, so that
currents from the polar regions would be very different from what
they are today. If, on the other hand, we may suppose that the rise of
"Gondwana Land" (from Brazil to India) was attended by the formation
of high mountains in those latitudes, we have the basis, at least, of
a more plausible explanation. Professor Chamberlin rejects this
supposition on the ground that the traces of ice-action are at or near
the sea-level, since we find with them beds containing marine fossils.
But this only shows, at the most, that the terminations of the glaciers
reached the sea. We know noth